Supercontinuum source, method for generating and emitting a supercontinuum, multiphoton excitation fluorescence microscope, and multiphoton excitation method

ABSTRACT

An ultrashort light pulse oscillated from an ultrashort pulse oscillator enters a waveguide  2  via a polarization control element  3.  After conversion into a supercontinuum by a nonlinear optical effect, it is compressed by a prism pair compressor  71  as pulse compressor  7,  and then emitted. The waveguide  2,  which is a nonlinear fiber with normal dispersion in the wavelength range from 850 to 1550, generates the supercontinuum having a spectrum continuous in a wavelength band width of at least 200 nm included in the wavelength range from 850 to 1550 nm. The supercontinuum, which has a peak power within 1 to 100 kW, can be used as excitation light in a multiphoton excitation fluorescence microscope for fluorescence observation of biological samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of Ser. No. 16/091,819filed Oct. 5, 2018, which is the U.S. National Phase Application ofInternational Application No. PCT/JP2017/014049, filed Apr. 4, 2017,which claims benefit of priority to Japanese Patent Application No.2016-178605, filed Sep. 13, 2016, and to Japanese Patent Application No.2016-076155, filed Apr. 5, 2016, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The invention of this application relates to generation of asupercontinuum, and relates to fluorescence microscopy.

TECHNICAL BACKGROUND

The phenomenon where a narrow-band ultrashort light pulse with high peakpower is spectrally broadened by nonlinear optical effects is known assupercontinuum. Patent Documents 1-5 disclose known supercontinuumsources as prior art.

A source to generate and emit a supercontinuum (supercontinuum source)includes a pulse oscillator to oscillate ultrashort light pulses, and anonlinear optical element. An ultrashort-pulse laser oscillator is oftenused as the pulse oscillator, and a waveguide type element such asnonlinear fiber is often used as the nonlinear optical element. Whenultrashort light pulses oscillated from the pulse oscillator enter intothe waveguide, it is spectrally broadened by nonlinear optical effectssuch as self-phase modulation, mutual phase modulation, four-wave mixingand Raman scattering, as it propagates through the waveguide, beingemitted as a supercontinuum.

PRIOR-ART REFERENCES Patent Documents

[Patent Document 1] JP, H10-90737, A

[Patent Document 2] JP, H11-174503, A

[Patent document 3] JP, 2003-149695, A

[Patent Document 4] JP, 2007-279704, A

[Patent Document 5] JP, 2008-216716, A

[Patent Document 6] JP, 2015-511312, A

Non-Patent Document

[Non-patent Document 1] Japanese Journal of Optics: a Publication of theOptical Society of Japan, the Japan Society of Applied Physics 44(1),11-17, 2015-01

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Expecting the practical application in optical communication mainly,researchers have been researching supercontinuums. To achieve highertransmission capacity, multicarrier (multiplex) transmission isrequired, and as a technique for this, adoption of a supercontinuum hasbeen considered. The practical application of supercontinuums, however,has not shown much progress, due to developments of other techniquessuch as multi-core structured optical fibers.

Although the situation is as described, supercontinuums have spectralcomponents in certain broadened ranges while characteristics as laserbeams are retained, and because of these superior characteristics,supercontinuums are assumed to be preferably applicable in other fields.

The invention of this application was invented in consideration of thesepoints, and has the object to provide a supercontinuum source emitting asupercontinuum usable in a new field.

Means for Solving the Problem

To achieve the described object, the invention claimed in the claim 1 ofthis application is a supercontinuum source to generate and emit asupercontinuum, comprising

-   -   a pulse oscillator oscillating an ultrashort light pulse, and    -   a waveguide converting the oscillated ultrashort light pulse        from the pulse oscillator into a supercontinuum by a nonlinear        optical effect, and then emitting,    -   wherein    -   the waveguide        -   converts the ultrashort light pulse into the supercontinuum            having a spectrum continuous in a wavelength band width of            at least 200 nm included in the wavelength range from 850 to            1550 nm, and        -   emits the supercontinuum enabling multiphoton excitation of            an irradiated object on an irradiated plane.

Furthermore, to achieve the described object, the invention claimed inthe claim 2 has the configuration where the pulse oscillator and thewaveguide emit the supercontinuum with a peak power within 1 to 100 kW,in addition to the configuration in the claim 1.

Furthermore, to achieve the described object, the invention the claimedin the claim 3 has the configuration to emit the supercontinuum havingthe wavelength flatness within 3 dB in a band width of at least 200 nmincluded in the wavelength range from 850 to 1550 nm, in addition to theconfiguration in the claim 1 or 2.

Furthermore, to achieve the described object, the invention claimed inthe claim 4 has the configuration that the pulse oscillator oscillatesthe ultrashort light pulse having a pulse width not more than 1 ps and acenter wavelength in the wavelength range from 1000 to 1100 nm, inaddition to the configuration in the claim 1, 2 or 3.

Furthermore, to achieve the described object, the invention claimed inthe claim 5 has the configuration that the waveguide converts theultrashort light pulse into the supercontinuum where the temporal shiftof wavelength is continuous, in addition to the configuration in theclaim 1, 2, 3 or 4.

Furthermore, to achieve the described object, the invention claimed inthe claim 6 has the configuration that the waveguide is a fiber withnormal dispersion in the wavelength range from 850 to 1550 nm, inaddition to the configuration in the claim 1, 2, 3, 4 or 5.

Furthermore, to achieve the described object, the invention claimed inthe claim 7 has the configuration that the center wavelength of theultrashort light pulse is in the range of plus and minus 50 nm to thepeak wavelength of the group velocity dispersion spectrum of the fiberwith normal dispersion, in addition to the configuration in the claim 6.

Furthermore, to achieve the described object, the invention as claimedin the claim 8 further comprising a pulse compressor to increase thepeak power of the supercontinuum by compressing the supercontinuumemitted from the waveguide, in addition to the configuration in theclaim 1, 2, 3, 4 or 5.

Furthermore, to achieve the described object, the invention claimed inthe claim 9 has the configuration comprising a pulse compressor toincrease the peak power of the supercontinuum by compressing thesupercontinuum emitted from the waveguide, in addition to theconfiguration in the claim 6 or 7.

Furthermore, to achieve the described object, the invention claimed inthe claim 10 is a supercontinuum source to generate and emitsupercontinuum, comprising

-   -   a pulse oscillator oscillating an ultrashort light pulse, and    -   a waveguide that converts the oscillated ultrashort light pulse        from the pulse oscillator into a supercontinuum by a nonlinear        optical effect and then emits    -   wherein    -   the waveguide converts the ultrashort light pulse into the        supercontinuum having a spectrum continuous in a band width of        at least 200 nm included in the wavelength range from 850 to        1550 nm, and    -   a pulse compressor is provided to increase the peak power of the        supercontinuum emitted from the waveguide by pulse compression        thereof.

Furthermore, to achieve the described object, the invention claimed inthe claim 11 is a method for generating and emitting a supercontinuum,comprising

-   -   oscillating an ultrashort light pulse by a pulse oscillator,    -   making the ultrashort light pulse from the pulse oscillator        enter into a waveguide, and    -   converting the ultrashort light pulse into a supercontinuum by a        nonlinear optical effect in the waveguide, and then emitting        therefrom,    -   wherein    -   the waveguide        -   converts the ultrashort light pulse into the supercontinuum            having a spectrum continuous in a band width of at least 200            nm included in the wavelength range from 850 to 1550 nm, and        -   emits the supercontinuum capable of multiphoton excitation            of an object on an irradiated plane.

Furthermore, to achieve the described object, the invention claimed inthe claim 12 further comprises emitting the supercontinuum with a peakpower within 1 to 100 kW, in addition to the configuration in the claim11.

Furthermore, to achieve the described object, the invention claimed inthe claim 13 further comprises emitting the supercontinuum having thewavelength flatness within 3 dB in a band width of at least 200 nmincluded in the wavelength range from 850 to 1550 nm, in addition to theconfiguration of the claim 11 or 12.

Furthermore, to achieve the described object, the invention claimed inthe claim 14 has the configuration where the ultrashort light pulseoscillated from the pulse oscillator has a pulse width not more than 1ps and the center wavelength in the wavelength range from 1000 nm to1100 nm, in addition to the configuration in the claim 11, 12 or 13.

Furthermore, to achieve the described object, the invention claimed inthe claim 15 has the configuration where the waveguide converts theultrashort light pulse into the supercontinuum where the temporal shiftof wavelength is continuous, in addition to the configuration in theclaim 11, 12, 13 or 14.

Furthermore, to achieve the described object, the invention claimed inthe claim 16 has the configuration where the waveguide is a fiber withnormal dispersion in the wavelength range from 850 to 1550 nm, inaddition to the configuration in the claim 11, 12, 13, 14 or 15.

Furthermore, to achieve the described object, the invention claimed inthe claim 17 has the configuration where the center wavelength of theultrashort light pulse is in the range of plus and minus 50 nm to thepeak wavelength of the group velocity dispersion spectrum of the fiberwith normal dispersion, in addition to the configuration in the claim16.

Furthermore, to achieve the described object, the invention claimed inthe claim 18 further comprises increasing the peak power of thesupercontinuum emitted from the waveguide by pulse compression thereof,in addition to the configuration in the claim 11, 12, 13, 14 or 15.

Furthermore, to achieve the described object, the invention claimed inthe claim 19 further comprises increasing the peak power of thesupercontinuum emitted from the waveguide by pulse compression thereof,in addition to the configuration in the claim 16 or 17.

Furthermore, to achieve the described object, the invention claimed inthe claim 20 is a method for generating and emitting a supercontinuum,comprising

-   -   oscillating an ultrashort light pulse by a pulse oscillator,    -   making the ultrashort light pulse from the pulse oscillator        enter into a waveguide, and    -   converting the ultrashort light pulse into a supercontinuum by a        nonlinear optical effect in the waveguide, and then emitting        therefrom,    -   wherein    -   the waveguide converts the ultrashort light pulse into the        supercontinuum having a spectrum continuous in a band width of        at least 200 nm included in the wavelength range from 850 to        1550 nm, and    -   the peak power of the supercontinuum is increased by compressing        the supercontinuum emitted from the waveguide.

Furthermore, to achieve the described object, the invention claimed inthe claim 21 is a multiphoton excitation fluorescence microscopecomprising the supercontinuum source claimed in the claim 1, 2, 3, 4, 5,6, 7, 8, 9 or 10, further comprising

-   -   an optical system to irradiate an object with the supercontinuum        emitted from the waveguide, and    -   a detector to detect a fluorescence emitted when the object is        subjected to multiphoton excitation by the supercontinuum.

The invention claimed in the claim 22 is a multiphoton excitation methodfor causing multiphoton excitation on a fluorescent substance by asupercontinuum, comprising

-   -   an oscillation step where an ultrashort light pulse is        oscillated by a pulse oscillator,    -   a conversion step where the ultrashort light pulse oscillated        from the pulse oscillator is made enter into the waveguide,        converted into the supercontinuum by a nonlinear optical effect        in the waveguide, and then emitted,    -   a compression step where the peak power of the emitted        supercontinuum is increased by pulse compression, and    -   an irradiation step where all or a part of the supercontinuum        which peak power has been increased in the compression step is        applied to the fluorescent substance to cause multiphoton        excitation,    -   wherein    -   the conversion step is the step where the ultrashort light pulse        is converted in the waveguide into the supercontinuum having a        spectrum continuous in a band width of at least 200 nm included        in the wavelength range from 850 to 1550 nm.

Effect of the Invention

As described later, according to the invention as claimed in the claim 1or 11, the source can be used preferably for fluorescence observation,measurement, and the like utilizing multiphoton excitation, because itemits a supercontinuum having a spectrum continuous in a band width atleast 200 nm included in the wavelength range from 850 to 1550 nm, andthe supercontinuum enables multiphoton excitation of an object.

According to the invention claimed in the claim 2 or 12, moreover, thesource can be used preferably for fluorescence observation of biologicalsamples, because the peak power is within 1 to 100 kW.

According to the invention claimed in the claim 3 or 13, moreover, thesource is more widely usable because of the wavelength flatness within 3dB.

According to the invention claimed in the claim 4 or 14, moreover, thehigh wavelength flatness can be easily achieved.

According to the invention claimed in the claim 5 or 15, moreover,because a linear chirp supercontinuum is generated, the characteristicis applicable to various uses.

According to the invention claimed in the claim 6 or 16, moreover, it iseasy to generate a supercontinuum in the wavelength range of 850-1550nm, or to generate a supercontinuum with higher peak power.

According to the invention claimed in the claim 7 or claim 17, moreover,it is possible to obtain a linearly chirped supercontinuum more easily,because the center wavelength of the ultrashort light pulse is withinthe range ±50 nm from the peak dispersion wavelength in the groupvelocity dispersion spectrum of the fiber with normal dispersion.

According to the invention claimed in the claim 8, 10, 18 or 20,moreover, it is possible to obtain a supercontinuum with high peakpower, because the peak power of the supercontinuum emitted from thewaveguide is increased by pulse compression.

According to the invention claimed in the claim 9 or claim 19, moreover,it is possible to obtain a supercontinuum with high peak power, becausethe peak power of the supercontinuum emitted from the waveguide isincreased by pulse compression. In this, it is possible to carry out thecompression more easily, because the supercontinuum is made linearlychirped by the fiber with normal dispersion and then subjected to thecompression.

According to the invention claimed in the claim 21, moreover, one laserenables multiphoton excitation simultaneously on two or more kinds offluorescent proteins having different absorption wavelengths in thewavelength range from 850 to 1550 nm. Providing a wavelength tuningmechanism, of course it is also possible to extract light at a desiredwavelength to observe fluorescence by monochromatic multiphotonexcitation. Whereas simultaneous polychromatic excitation needed two ormore lasers conventionally, it is possible to present a space-savingwidely-usable multiphoton excitation fluorescence microscope accordingto the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a supercontinuum source in a firstembodiment.

FIG. 2 is the figure showing an analysis result by a spectrum analyzerfor light emitted from an example of the supercontinuum source shown inFIG. 1 (Example 1).

FIG. 3 is the figure showing other analysis results for lights emittedfrom supercontinuum generation sources as other examples (Example 2 andExample 3).

FIG. 4 is the figure showing the spectral dispersion characteristic of awaveguide provided in the supercontinuum source in the embodiment.

FIG. 5 is the figure showing the spectral dispersion characteristic of anonlinear fiber used as the waveguide in a referential example.

FIG. 6 is the figure showing an analysis result of the spectrum of asupercontinuum generated on the configuration of the referentialexample.

FIG. 7 is a schematic view of a multiphoton excitation fluorescencemicroscope in an embodiment.

FIG. 8 is the figure showing a spectrogram of a supercontinuum emittedfrom the supercontinuum source in Example 1.

FIG. 9 is the figure showing a spectrogram of a supercontinuum emittedfrom the supercontinuum source in the referential example.

FIG. 10 is a schematic view of a supercontinuum source in a secondembodiment.

FIG. 11 is the figure schematically showing function and operation ofthe supercontinuum source in the second embodiment.

FIG. 12 is a schematic view of a pulse compressor used for thesupercontinuum source in the second embodiment.

FIG. 13 is the figure showing the result of an experiment where increaseof a peak power in the second embodiment was confirmed.

FIG. 14 is the figure showing the supercontinuum of FIG. 13 as aspectrogram.

FIG. 15 is a schematic view showing other configuration examples forlight extraction.

FIG. 16 is a schematic view showing another configuration for the pulsecompressor.

FIG. 17 is a schematic view showing a further configuration for thepulse compressor.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are described as follows. FIG. 1 isa schematic view of the supercontinuum source in the first embodiment.Operation of the supercontinuum source in the embodiment is included inthe following description. This corresponds to description on asupercontinuum generation method in an embodiment of the invention.

The major feature of the supercontinuum source in this embodiment isthat it emits a supercontinuum with the photon density not less than theminimum density capable of multiphoton excitation of an object. That is,the supercontinuum source in the embodiment is configured as a lightsource for multiphoton excitation.

A multiphoton excitation fluorescence microscope is assumed as apreferable application of this supercontinuum source.

Fluorescent microscopy is the type of microscopy where an object isobserved by catching fluorescent emission from the object irradiated toexcitation light. It has been the important technique especially in suchfields as biotechnology research and biological sample observation.Fluorescent microscopes have the features that only a part of an objectcan be observed by dying it, and that the activity of a cell can beobserved as alive.

In these years, the technique of two-photon excitation fluorescencemicroscopy has been developed in fluorescence microscopy. Whilefluorescence emission by light excitation is usually caused withabsorption of one photon into a molecule, two photons are simultaneouslyabsorbed to excite and then causes a fluorescence in the two-photonexcitation. Theoretically it is possible to make three or more photonsabsorbed simultaneously for excitation. The fluorescent microscopymaking two or more photons absorbed simultaneously for excitation iscalled multiphoton excitation fluorescence microscopy. The detailedconfiguration of a multiphoton excitation fluorescence microscope isdisclosed in Non-patent Document 1 for instance.

The multiphoton excitation microscopy is advantageous in several pointscompared to other kinds of fluorescence microscopy. One of those iscapability of easy wavelength separation. In fluorescence microscopy, itis required to separate a fluorescence from light at the excitationwavelength to observe only the fluorescence. The wavelength of afluorescence, however, is often close to the excitation wavelength,being difficult to separate, in the normal fluorescence microscopy,i.e., one photon excitation fluorescence microscopy. Such an element assharp-cut filter may be used to eliminate the excitation light toextract only a fluorescence. However, the wavelength band of afluorescence often overlaps with the wavelength band of the excitationlight, and the fluorescence might often be eliminated in eliminating theexcitation light. As a result, it is impossible to catch thefluorescence with sufficient intensity.

In two-photon excitation, on the other hand, the separation is madeeasier by such an element as sharp-cut filter, and it is possible toobserve a fluorescence without losing, because the excitation wavelengthis two times longer than the wavelength of the fluorescence.

Another advantage of the multiphoton excitation fluorescence microscopyis that fluorescence observation is enabled at any position in depthdirection. In multiphoton excitation, two or more photons need tocollide with a fluorescent molecule within a very short period(approximately 10⁻¹⁶ seconds, substantially at the same time), andtherefore a very high photon density is required. When excitation lightis concentrated in an object transparent to the excitation light to someextent, therefore, multiphoton excitation is caused only at theconcentration point, and thus only the fluorescence emission at thepoint can be observed. Moreover, shift of the concentration positionenables observation at any desired position in the depth direction. Itconsequently has the pinhole effect as in confocal microscopes, causingno loss of fluorescence emission either because no pinhole is required.This is the merit thereof especially, for instance, when the inside of abiological sample is to be observed.

To achieve multiphoton excitation, it is necessary to increase photondensity temporally and spatially. Therefore, an ultrashort pulse laseroscillator, which emits a pulse of light within a very short period,e.g., femtosecond order, is employed as a light source. Total powergenerated on the laser oscillator is concentrated in a short period,thereby achieving a high photon density required for multiphotonexcitation.

Due to some problems, the multiphoton excitation fluorescence microscopyas described has not spread widely, only being partly put to practicaluse. One reason of this is that degree of freedom in the observationcondition is extremely low.

As well known, depending on fluorescent molecules, excitation wavelengthis different, and wavelength of fluorescence emission is also different.In observing a different kind of samples, therefore, it is necessary toirradiate them to excitation light at a different wavelength in almostall cases. For conventional multiphoton excitation fluorescencemicroscopes, because ultrashort pulse laser oscillators with differentemission wavelengths are needed to apply excitation lights at differentwavelengths, one ultrashort pulse laser oscillator is needed logicallyfor each kind of fluorescent molecules to observe. As well known,ultrashort-pulse laser oscillators are very expensive devices, andtherefore it is far beyond the feasible range to prepare each individualultrashort pulse laser oscillator for each different kind of fluorescentmolecules.

Patent Document 6 discloses a multiphoton excitation fluorescencemicroscope that employs a couple of ultrashort pulse laser oscillatorsto observe three different-kind fluorescent molecules. According to thisdisclosure, though the number of the ultrashort pulse lasers can be lessthan the number of kinds of fluorescent molecules to observe, a coupleof ultrashort pulse laser oscillators are still necessary, and thus thehigher cost is not a small problem.

In the multiphoton excitation fluorescence microscope in Patent Document6, moreover, it is necessary to overlap temporally and spatiallyultrashort pulse laser beams from the couple of ultrashort pulse laseroscillators within the concentration spot. Requiring a much-complicatedlarge-in-scale mechanism and control system, it raises the problem ofdifficulty in the adjustment work in addition to the cost problem.

Some practically usable multiphoton excitation fluorescence microscopesemploy the configuration where a wavelength tuning mechanism isinstalled to an ultrashort pulse laser oscillator, i.e., tunable laserconfiguration. This kind of tuning mechanism, however, uses a nonlinearoptical element such as OPO (optical parametric oscillator), having theproblem of requiring a troublesome work for wavelength adjustment suchas phase matching.

The inventor assumed that each described problem for multiphotonexcitation fluorescence microscopes could be solved by adoptingsupercontinuum, and diligently carried out a research. Thesupercontinuum source in the embodiment is based on the result of thisresearch.

Concretely, the supercontinuum source in the embodiment shown in FIG. 1includes a pulse oscillator 1 oscillating ultrashort pulses of light,and a waveguide 2 to cause a nonlinear optical effect. An ultrashortpulse laser is used for as the pulse oscillator 1. This ultrashort pulselaser oscillates ultrashort pulses of the femtosecond order, andpreferably the pulse width is 1 ps (picosecond) or less. The centerwavelength of the ultrashort pulse laser is preferably within about1000-1100 nm. Specifically, it may be an ultrashort pulse fiber laseremploying a rare-earth, e.g., Yb, doped fiber, or ultrashort pulse laseremploying a Yb-based crystal, e.g., Yb:KYW excited by a laser diode.

The waveguide 2 is to generate a supercontinuum from the ultrashortlight pulse by a nonlinear optical effect, i.e., nonlinear waveguide. Inthis embodiment, an optical fiber is used as the waveguide 2, which ispreferably a highly nonlinear fiber. The nonlinear fiber as thewaveguide 2 may be a loop-shaped flexible one, or rod-shaped one. Inthis embodiment, a polarization control element 3 is provided betweenthe pulse oscillator 1 and the waveguide 2. The polarization controlelement 3 is to enhance the nonlinear optical effect on the waveguide 2,and to make the ultrashort pulses of light polarized according to thecharacteristics of the waveguide 2 before entering thereto. A wavelengthplate, e.g., ½- or ¼-wavelength plate, is used as the polarizationcontrol element 3. In this embodiment, the polarization control element3 makes the ultrashort pulses of light linearly polarized in a desireddirection according to the characteristic of the waveguide 2. Linearpolarization is just an example, not limiting the invention.

The supercontinuum source in this embodiment is the one to emit asupercontinuum having a spectrum continuous within a band at least 200nm included in the wavelength range of 850-1550 nm. The supercontinuumsource in this embodiment also emits a supercontinuum which temporalpeak power is preferably 1 kW or more. These points are described with amore specific example.

FIG. 2 is the figure showing an analysis result by a spectrum analyzerfor light emitted from the supercontinuum source in the embodiment shownin FIG. 1 (Example 1). FIG. 2(1) is the figure showing a pulse shape,i.e., variation of emission intensity (wavelength-integrated intensity)as time passes. FIG. 2(2) is the figure showing the spectraldistribution of the emitted light with the pulse shape of (1), which isenergy per wavelength at each wavelength in one pulse.

This example employs a pulse laser having a Yb-base crystal emittinglight pulses of 1045 nm in the center wavelength and 200 fs in the pulsewidth, as the pulse oscillator 1. As shown in FIG. 2(1), the temporalpeak power of a light pulse emitted from the waveguide 2 isapproximately 2 kW. As shown in FIG. 2(2), the emitted light pulse iscontinuous within the range of about 800-1400 nm, and thussupercontinuum generation was confirmed.

In the supercontinuum source in the embodiment, wavelength flatness ofthe supercontinuum emission is 3 dB or less. The wavelength flatnessmeans intensity uniformity among spectral components, in other words,smallness of spectral intensity unevenness. “3 dB or less” means thatintensity at the highest-intensity wavelength is within 3 dB to theintensity at the lowest-intensity wavelength. As shown in FIG. 2(2), thesupercontinuum by the supercontinuum source in Example 1 has theintensity unevenness within 3 dB in the range about 850 to 1250 nm.

The within-3 dB wavelength flatness is described in more detail withother examples. FIG. 3 is the figure showing analysis results of lightsemitted from supercontinuum sources as other two examples (example 2 andexample 3).

FIG. 3(1) shows Example 2 where an optical parametric amplified system(OPA) of 1100 nm in the center wavelength and 1 ps in the pulse widthwas employed for the pulse oscillator 1. In this example, the wavelengthflatness within 3 dB in the wavelength range about 850-1550 nm wasobtained.

FIG. 3(2) further shows Example 3 where a Yb-doped fiber laseroscillator of 1030 nm in the center wavelength and 170 fs in the pulsewidth 170 was employed for the pulse oscillator 1. In this example aswell, the wavelength flatness within 3 dB in the wavelength range about950-1150 nm was obtained.

The described superior performance of the supercontinuum source in theembodiment, that is, the high wavelength flatness, is much affected bythe characteristics of the waveguide 2. This point is described asfollows.

As the waveguide 2, the supercontinuum source in the embodiment employsa nonlinear fiber with normal dispersion in the range of emissionwavelengths. FIG. 4 is the figure showing the spectral dispersioncharacteristic of the waveguide 2 for the supercontinuum source in theembodiment.

In the field of optical communication, it is important to reduce groupvelocity dispersion (GVD), which is one of fundamental characteristicsfor optical fibers. Conversely, a nonlinear optical fiber used forsupercontinuum generation enhances group velocity dispersion by anonlinear optical effect, and thereby broadens a band width. As groupvelocity dispersion characteristics, normal dispersion and anomalousdispersion are known.

In these, the nonlinear fiber used as the waveguide 2 in the embodimenthas normal dispersion as shown in FIG. 4. That is, GVD of this waveguide2 is negative within the range of 800-1600 nm. The nonlinear fiber inthis example has the maximum GVD, which is approximately −30 ps/nm/km,at around 1000-1100 nm. Such a nonlinear fiber is being sold asNL-1050-NEG-1 by NKT Photonics A/S, Denmark, being available therefrom.In Example 1, length of the nonlinear fiber used as the waveguide 2 is500 mm.

Using the waveguide 2 with normal dispersion in the emission wavelengthrange as described, the supercontinuum has preferable wavelengthcomponents with neither spectral vacancy nor extreme spikes(ultra-narrow-band intense waves). The following is the description onthe result of a comparative experiment where this point was confirmed.

In this experiment, a nonlinear fiber not having normal dispersion inthe wavelength range of 850-1550 nm was used as the waveguide 2 insteadof the nonlinear fiber used in described Example 1, and similarly asupercontinuum was generated to analyze the spectrum thereof. Conditionsother than use of the nonlinear fiber not having normal dispersion werethe same as in Example 1. “Not having normal dispersion in thewavelength range of 850-1550 nm” means that dispersion is anomalous in apart or all of the wavelength range of 850-1550 nm.

FIG. 5 is the figure showing the spectral dispersion characteristic ofthe nonlinear fiber used as the waveguide 2 in the referential example.As show in FIG. 5, GVD of the nonlinear optical fiber used as thewaveguide 2 in the referential example is positive (anomalous) in thewavelength range longer than 1040 nm. More specifically, a nonlinearfiber SC-5.0-1040 sold by NKT Photonics A/S was used for the waveguide 2in the referential example. This fiber was 1000 mm in length.

FIG. 6 is the figure showing the analysis result for the spectrum of asupercontinuum generated in the referential example. As well as FIG. 2,FIG. 6(1) shows variation of emission intensity (wavelength-integratedintensity) as time passes, and FIG. 6(2) is the figure showingtime-integrated energy per wavelength at each wavelength in one pulse.

In the referential example, as shown in FIG. 6(2), the observed spectrumof the supercontinuum includes a lot of spike-shaped peaks, not being aflat distribution. That is, the wavelength flatness of 3 dB or less isnot achieved in at least 950-1150 nm. The reason of this is supposedlyas follows. Because GVD is positive, the propagation velocity of alonger wavelength wave, which has generated initially (early) in anultrashort pulse, is delayed, and the propagation velocity of a shorterwavelength wave, which has generated later therein, is enhanced. As aresult, nonlinear optical effects such as self-steepening, induced Ramanscattering and optical soliton are caused. Thus, the spectrum of thesupercontinuum may include a lot of spikes as shown in FIG. 6(2), thoughit has become wider in the wavelength band. As shown in FIG. 2(2), bycontrast, the supercontinuum having the high wavelength flatness with nospike noises is obtained when the nonlinear fiber with normal dispersionis used as the waveguide 2.

As described, the supercontinuum source in the embodiment can bepreferably used for fluorescence observation and measurement of anobject utilizing multiphoton excitation, because the supercontinuum iscontinuous in at least 200 nm band width included in the wavelengthrange from 850 to 1550 nm, and because the supercontinuum is capable ofcarrying out multiphoton excitation on an object.

Moreover, because of the wavelength flatness within 3 dB, the source isavailable for wider uses. In addition, since the nonlinear fiber withnormal dispersion is used as the waveguide 2, the high wavelengthflatness can be easily obtained. When a nonlinear fiber not havingnormal dispersion is used as the waveguide 2, it is necessary to controlor adjust higher-order nonlinear optical effects to obtain the highwavelength flatness. However, this is very difficult.

The point that the pulse oscillator emits ultrashort light pulses of1000 to 1100 nm in the center wavelength and 1 ps in the pulse widthbrings about the effects that a supercontinuum can be easily generatedin the wavelength band of 850-1550 nm, and that a supercontinuum havingthe high peak power can be easily generated. That is, when the pulsewidth of the ultrashort light pulses is longer than 1 ps, thesupercontinuum cannot have a wide-band spectrum. When the centerwavelength of the ultrashort light pulses is in the range of 1000-1100nm, the supercontinuum easily can have a spectrum broadened over atleast 200 nm in the range of 850-1550 nm by a nonlinear optical effect.

A multiphoton excitation fluorescence microscope according to anembodiment of the invention is described next.

FIG. 7 is a schematic view of the multiphoton excitation fluorescencemicroscope in the embodiment. The multiphoton excitation fluorescencemicroscope shown in FIG. 7 includes a supercontinuum source 4, anoptical system 5 to irradiate an object S with a supercontinuum emittedfrom the supercontinuum source 4, and a detector 6 to detect afluorescence emitted from the object S excited by the supercontinuumirradiation. The supercontinuum source 4 is the described one having thepulse oscillator 1, the polarized-light control element 3, and thewaveguide 2.

The optical system 5 includes several lenses 51, a dichroic mirror 52, ascanning mirror unit 53, and an objective lens 54 in this embodiment.The lenses 51 are disposed to modify the beam pattern of the emittedsupercontinuum and/or to change the beam size thereof. The dichroicmirror 52 is disposed to separate the supercontinuum as excitation lightand a fluorescence emitted by the excitation.

The objective lens 54 is disposed to concentrate the supercontinuum tothe object S. In particular, the objective lens 54 is capable ofconcentrating the supercontinuum to a desired depth position in theobject S that could be a biological sample.

The multiphoton excitation fluorescence microscope in the embodiment isa kind of laser scanning microscopes, including the scanning mirror unit53 to scan the light concentration point on an observation planevertical to the optical axis. The scanning mirror unit 53 furtherincludes a pair of scanning mirrors 531, to which a drive mechanism (notshown in the figure) is provided to change the posture of each scanningmirror 531 independently. The light concentration point by the objectivelens 54 is scanned on an X-Y plane vertical to the optical axis when thedrive mechanism drives each scanning mirror 531.

A photo-multiplier or avalanche photodiode, etc. is used as the detector6. The imaging device 61 is the device that memorizes the output fromthe detector 6, obtain a 2D image therefrom or 3D image by overlaying 2Dimages, and displays the image as an observation result.

The imaging device 61 memorizes the output from the detector 6 in amemory (RAM) synchronously with the scanning by the scanning mirror unit53. The output includes each intensity signal of fluorescence emittedfrom each concentration point. Thus, an image is visualized in one framewith the contrast corresponding to the intensity signals. When theobjective lens 54 is operated to change the light concentration point,because it is on another observation plan, a 3D observation image ismade by memorizing it as new image data and integrating it with otherimage data on other observation planes. So as to carry out the dataprocessing as described, the imaging device 61 includes a processor anda program executable by the processor, and includes a display to displaythe images.

A filter 62 to extract only a fluorescence to observe is disposed on theoptical path at the incident side to the detector 6. The filter 62 maybe a sharp cut filter, band-bus filter or the like.

A pinhole board may be disposed on the optical path at the incident sideto the detector 6. The pinhole is located at the focus by the objectivelens 54 as well as the object S, i.e., made confocal, establishing theconfiguration of so-called confocal laser scanning microscope. Thissolution enables to obtain a clear image where noises in the depthdirection are eliminated.

In the multiphoton excitation fluorescence microscope in the embodiment,the supercontinuum source 4 enables observation of multiphoton-excitedfluorescence in an object. This point is described as follows.

Whether observation of multiphoton-excited fluorescence is possible ornot is whether a fluorescence can be emitted by multiphoton excitation,and whether it has a sufficient intensity for observation (i.e., forimaging) on a microscope. Accordingly it depends on the photon densityof excitation light in the end. Actually, because the objective lens 54concentrates excitation light to one point, which corresponds to onepixel in imaging, it is whether the number or density of photons at thepoint is not less than the amount capable of multiphoton-excitedfluorescence observation. Though theoretically excitation by three ormore photons is possible, two-photon excitation is taken.

In the following description, excitation of EGFP (enhanced greenfluorescent protein) by light having the wavelength of 1000 nm is takenas an example.

Showing the condition of EGFP as object S, the EGFP density (expressiondensity) in a cell is assumed to be 1×10⁻⁵M, and refractive index of asolution in which the cell including EGFP is dipped is assumed to be1.47. Quantum efficiency of EGFP is assumed to be 0.6.

On the other hand, capability of multiphoton-excited fluorescenceobservation means that a fluorescence (usually having the wavelengthhalf of the excitation light) by multiphoton excitation is caught on thedetector 6, and converted into electric signals making an observableimage. Therefore, it is necessary to consider the conditions of theoptical system 5 and the detection system.

The size of an X-Y plane (observation plane) in which the scanningmirror unit 53 carries out the scanning is assumed to be, for example,512×512 pixels. This means excitation light is concentrated at each of512×512 points to emit a fluorescence by two-photon excitation. In this,the size of one pixel, which corresponds to the beam diameter at thelight concentration point by the objective lens, is assumed to be 1 μmin diameter. The number of gray levels (grayscale) at each pixel, whichaffects the number of photons necessary, is assumed to be 12 bits.

How long one point should be irradiated to excitation light depends onhow long the frame period is, i.e., length of the period where one framewith 512×512 pixels for one observation plane is observed. Though theframe period is preferably short when motion of a biological sample isobserved, fluorescence observation could not be carried out sufficientlyin a shorter frame period, because only the smaller amount offluorescence is emitted from one point to be caught on the detector 6.In consideration of these, the frame rate is assumed to be, for example,5 fps (i.e., five frames per second).

Showing conditions of the optical system 5 and the detection system, 30%of light emitted from the supercontinuum source 4 is assumedly collectedby the optical system 5 for irradiation of EGFP, and photoelectricconversion efficiency of the photo-multiplier used as the detector 6 isassumed to be 40%.

Though the specific calculating formulae are omitted, the number ofphotons needed at one pixel (one light concentration point by theobjective lens 54) was calculated to be 8192 according to the describedconditions.

On the other hand, if it is assumed that a 1 kW-output supercontinuumsource 4 is used under the described conditions, the number of effectivephotons was 10112. In this, the effective photons means photons that areapplied to EGFP, causes fluorescence and are caught in the detector 6,i.e., converted into an electric signal. Thus, the excess over therequired number was confirmed. The 1 kw output in this means that atemporal peak power in one pulse is 1 kW or more, and 1 kW or more at awavelength of two-photon excitation. The photon density required in onepixel would be about 8192×10¹²/m² or more, when the supercontinuumirradiation diameter is 1 μm and when the pixel square size is 1 μm (1μm×1 μm), as described. In this case, when the number of photonsobtained in the 1 μm irradiation diameter is 10112, the photon densitywould be about 12882×10¹²/m², and thus the required photon density isachieved.

Though EGFP was adopted as an example in the above description, for manyother fluorescent materials it is possible to know the required numberof photons by an adequate calculation according to each condition ofexpression density, refractive index of dipping solution and quantumefficiency, and it is possible to know the output (peak power) of thesupercontinuum source 4 in order that the number of photons exceeds it.Though detailed description is omitted, 1 kW or more enables irradiationof excitation light with the number of photons not less than the minimumnumber required for multiphoton-excited fluorescence observation foralmost all kinds of biological samples, being preferable for multiphotonexcitation observation of biological samples. The output exceeding 100kW may damage biological samples in many cases. Therefore, the outputfrom 1 to 100 kW is preferable as the supercontinuum source formultiphoton-excited fluorescence observation of biological samples.

In the above discussion on the required number of photons, though therequired peak power was the peak power at the wavelength of excitationlight, it is necessary that the temporal peak power provides the numberof photons not less than the required minimum number in a certainwavelength range, because excitation light would be changed according tokinds of fluorescent molecules to observe. The following is asupplementary description on this.

FIG. 8 and FIG. 9 are the figures showing the spectrograms ofsupercontinuums emitted from the supercontinuum source in Example 1 andthe supercontinuum source in the referential example. FIG. 8 shows ofExample 1, and FIG. 8 shows of the referential example. Though theintensities were expressed by color in the original spectrograms, thoseare shown in the lower areas in FIG. 8 and FIG. 9 respectively, becausethe figures are black-and-white.

As recognized in FIG. 8, the supercontinuum emitted from thesupercontinuum source in Example 1 is at a longer wavelength initiallyin one pulse, and then the emission shifts to a shorter wavelengthgradually and continuously as time passes, that is, having a spectrumcontinuous temporally as well. This example, which has the power of 1 kWor more within the range from about 850 to 1550 nm, is evaluated aspreferable for multiphoton excitation fluorescence microscopes utilizingexcitation light in this wavelength range.

By contrast, as shown in FIG. 9, the spectrum of the supercontinuumemitted from the supercontinuum source in the referential example is notcontinuous, that is, has a spectral vacancy, though the whole wavelengthrange is broadened. If a supercontinuum includes a spectral vacancy, itis not suitable for multiphoton excitation fluorescence microscopes,because a fluorescent molecule to be excited in the vacant band cannotbe observed.

According to the multiphoton excitation fluorescence microscope in thedescribed embodiment, even without a wavelength tuning mechanism, it ispossible to carry out fluorescence observation as an object is excitedby excitation light having a desired wavelength in a band width at least200 nm included in the wavelength range from 850 to 1550 nm. Thismultiphoton excitation fluorescence microscope, therefore, has widerusability.

Though EGFP was adopted as fluorescent molecule in the abovedescription, multiphoton-excited fluorescence observation is possible inthe same manner for other kinds of natural or synthetic fluorescentmolecules. The supercontinuum source 4 in the embodiment can bepreferably used also for fluorescence observation where objects S arevarious kinds of materials other than biological samples. Processesutilizing multiphoton excitation other than fluorescence microscopy areknown (e.g., measurement of multiphoton excitation spectra), and thesupercontinuum source 4 in the embodiment also can be used for suchother uses.

The supercontinuum source in the second embodiment is described next.FIG. 10 is a schematic view of the supercontinuum source in the secondembodiment.

As understood in comparison of FIG. 1 and FIG. 10, the supercontinuumsource in the second embodiment has the configuration where a pulsecompressor 7 is provided on the emission side to the waveguide 2. FIG.11 is the figure schematically showing function and operation of thesupercontinuum source in the second embodiment. The upper part in FIG.11 schematically shows how the pulse shape is transformed in the secondembodiment, and the lower part schematically shows how the spectralprofile is transformed.

In the second embodiment, as shown in FIG. 11, the output of the pulseoscillator is, for instance, less than 250 fs in the pulse width, 1045nm in the center wavelength, and 16 nm or less in the FWHM (full widthat the half maximum). This output is transformed into a supercontinuumhaving the pulse width of about 400-500 nm by utilizing self-phasemodulation in a normal dispersion fiber. In this, the pulse widthextends to about several picoseconds. The pulse compressor 7 compressesthe pulses as broadness of the wavelength band width is maintained, andthen makes the pulse width less than 1 ps.

FIG. 12 is a schematic view of the pulse compressor 7 used for thesupercontinuum source in the second embodiment. After reaching theconcept of the supercontinuum source in the first embodiment, theinventors had the idea to add the pulse compressor 7. As described, theinventors intended to obtain a supercontinuum with the spectrumcontinuous in a band width of at least 200 nm in the wavelength rangefrom 850 to 1550 nm so that it can be used preferably for multiphotonexcitation fluorescence microscopes or other uses, and employed anonlinear fiber with normal dispersion as one specific means to achievethis. By this, the spectrum of the supercontinuum is made flat andcontinuous in the wavelength band. As a result, the supercontinuum withthe spectrum continuous also temporally is generated as shown in FIG.8(1).

A light pulse having a spectrum continuous also temporally as shown inFIG. 8(1) is called “chirped pulse”. To obtain a chirped pulse, usuallyan element with linear group-delay dispersion is used. By causing timelags corresponding to wavelength components originally existing in apulse, a chirped pulse is obtained.

By contrast, the described first embodiment newly generates wavelengthcomponents not originally existing by nonlinear optical effects such asself-phase modulation, and then obtains a spectrum continuous andwithout spectral vacancy over a broader band width of 200 nm or more. Inthis, consequently the obtained spectrum is continuous also temporallyas shown in FIG. 8(1). That is, a chirped pulse is obtained. The pulseshown in FIG. 8(1) can be called “linearly chirped pulse”. Linear chirpmeans that wavelength continuously shifts with time. As shown in FIG.8(1) is, furthermore, it is the chirped pulse having no extremal values(i.e., neither local maximums nor local minimums) in the wavelengthshift.

Seeing the performance of the first embodiment, the inventors had theidea of applying pulse compression to the obtained supercontinuum. Thereason of this is that pulse compression is more preferable for such ause as multiphoton excitation because peak power increases, andespecially that it is easily applicable to such linearly chirped pulsesas shown in FIG. 8(1).

In the second embodiment, more concretely, a prism-pair compressor 71 isused as the pulse compressor 7, as shown in FIG. 12. In this example,the compressor includes two pairs of (four in total) prisms 711

In a supercontinuum emitted from the waveguide 2, a wave existing at anearlier time in one pulse has a longer wavelength, and another waveexisting at a later time has a shorter wavelength, as shown in FIG.8(1). That is, it undergoes a spectral shift to shorter wavelengths astime passes. Pulse compression is applied easily to such a chirped pulseby a dispersion element where GDD (group delay dispersion) increaseslinearly as wavelength increases. The prism pair compressor 71 in FIG.12 is the element to carry out this kind of dispersion. A return mirror712 is disposed to two pairs of prisms 711, and therefore a light beampasses through the prisms 711 eight times in total.

As shown in FIG. 12, each prism 711 is a triangular prism, extendingalong the direction perpendicular to the plane of FIG. 12. Two pairs ofthe prisms 711 are symmetric to a plane (shown with “S” in FIG. 12)perpendicular to the figure plane.

As shown in FIG. 12, a light wave L₂ at a longer wavelength has a longeroverall propagation length through the four prisms 711, a light wave ata shorter wavelength has a shorter overall propagation length, and thenthe light wave L₁ at the shortest wavelength has the shortest one. Thus,a light wave at a longer wavelength is delayed more, whereas a lightwave at a shorter wavelength is delayed less. When the size and thedisposition interval of the prisms 711 are adequately chosen accordingto the refractive index of the material of the prisms 711, therefore,each wavelength in a pulse of the supercontinuum having returned afterpassing through the prisms 711 eight times would be temporally aligned(coherent), and thus the pulse is compressed.

The described excellent pulse compression property is due to theoriginal characteristic of light, i.e., linear chirp. The linear chirpitself is due to the correspondence of the wavelengths, i.e., thecorrespondence of the center wavelength of ultrashort light pulses andthe peak wavelength in the GVD spectrum of the normal dispersion fiber.Linear chirped pulses are obtained because the group velocity isdispersed uniformly to the shorter wavelength side and the longerwavelength side from the center wavelength of the ultrashort lightpulses. According to the research by the inventors, the centerwavelength of the ultrashort light pulses may be within ±50 nm from thepeak wavelength in the GVD spectrum of the fiber with normal dispersion,in order to obtain linearly chirped pulses easily.

Showing a more specific design example of the prism pair compressor 71,dispersion at 1000 nm would be about −822 fs², when each prism 711 ismade of quartz (refraction index 1.45 at the wavelength 1000 nm), thetop angle of each prism 711 is 69.1°, the disposition interval (shownwith “L” in FIG. 12) is 1 m, and d1+d2 as the light incident position is10 cm. Because the center of the spectrum is approximately 1000 nm, theincident angle θ to the prisms 711 is the Brewster's angle at 1000 nm,i.e., 55.4°.

The peak power of the supercontinuum compressed as described would behigher, because the waves are overlapped within an extremely narrow timeband. This situation is shown in FIG. 13. FIG. 13 is the figure showingthe result of an experiment where an increase of the peak power in thesecond embodiment was confirmed.

In this the experiment, the supercontinuum shown in FIG. 8 wascompressed with two pairs of prisms 711 as shown in FIG. 11. While FIG.13(1) shows a couple of pulse shapes at the all wavelengths included,one before the compression is depicted with a dotted line (the sameshape in the lower part of FIG. 8), and the other one after thecompression is depicted with a solid line. While FIG. 13(2) shows acouple of the spectral distributions, one before the compression isdepicted with a dotted line, and the other one after the compression isdepicted with a solid line.

The horizontal axis in FIG. 13(1) is time (ps), and the vertical axis ispower (kW). The horizontal axis in FIG. 13(2) is wavelength, and thevertical axis is energy (pJ/nm) normalized by wavelength, i.e., energyper wavelength.

As shown in FIG. 13(1), the supercontinuum is subjected to pulsecompression as passes through the prism pair compressor 71. The FWHM ofthe pulse was about 1 ps after the compression, whereas it was about 4ps before the compression. Then the peak power increases two times ormore by the compression. Moreover, the dotted line and the solid linealmost completely overlap in FIG. 13(2), showing almost no change of thespectral profile before and after the compression. That is, the spectralcontinuity over the range of about 850-1350 nm is maintained even afterthe compression.

Though FIG. 13(2), where the vertical axis is time-integrated energy(pJ), shows no change of time-integrated energy of light at eachwavelength within the pulse, each instantaneous peak value at eachwavelength has increased because the pulse width has been narrowed.Situation of the increase is similar as of the power in the wholewavelength range shown in FIG. 13(1).

FIG. 14 is the figure where the supercontinuum of FIG. 13 is shown as aspectrogram. Because the original is a color image, the pulse shape isshown in the lower area thereof as well as in FIG. 8.

As well understand by comparing FIG. 8 and FIG. 14, each wave at eachwavelength overlaps at almost the same time by the pulse compression.The peak power increases greatly as a result of this overlapping.

As described, since the supercontinuum source in the second embodimentincludes the pulse compressor 7 that compresses a supercontinuum emittedfrom the waveguide 2, the peak power is increased. Therefore, it is morepreferable for such an application as multiphoton excitationfluorescence microscopy. That is, it can cause multiphoton excitationeasily because of the higher peak power, and cause less damage to anobject because of the narrower pulse width. It is necessary that manyphotons exist at the same time (or within a very narrow time band) formultiphoton excitation, and therefore the high peak power isadvantageous. On the other hand, thermal damage to objects depends onirradiation dose, i.e., time-integrated amount of irradiation.Therefore, a light pulse with a narrow pulse width and high peak poweris especially preferable in observing a low-heat-tolerance object, e.g.,biological sample, by multiphoton excitation. For instance, it ispossible to observe kinds of fluorescent proteins without color fading.

The supercontinuum source in the second embodiment needs a configurationto extract compressed light pluses from the optical path. From somepossible solutions, a configuration using a polarization beam splitter713 is adopted in the example shown in FIG. 12.

Concretely, the polarization beam splitter 713 is disposed on theoptical path from the waveguide 2. A light beam emitted from thewaveguide 2 enters the polarization beam splitter 713. Aquarter-wavelength plate 714 is disposed between the polarization beamsplitter 713 and the pulse compressor 7. As described, thesupercontinuum source in the embodiment comprises the polarizationcontrol element 3 to emit a linearly polarized supercontinuum from thewaveguide 2. If a supercontinuum emitted from the waveguide 2 is notlinearly polarized, an adequate polarization control element is disposedbetween the waveguide 2 and the polarization beam splitter 713 to makeit linearly polarized.

The supercontinuum P1 from the waveguide 2 passes through thepolarization beam splitter 713, becomes the circular polarized light P2on the quarter-wavelength plate 714, and returns to thequarter-wavelength plate 714 after being subjected to pulse compressionby the prisms 711 as described. Then, it becomes the linearly polarizedlight P3, which polarization direction is 180° different, on thequarter-wavelength plate 714, and reaches the polarization beam splitter713. Further, it is extracted from the optical path by a reflection inthe polarization beam splitter 713, and then guided to a desired placeto be utilized.

Instead of the configuration to extract a supercontinuum by thepolarization beam splitter 713, the supercontinuum source may often havea configuration of oblique optical paths, otherwise a configuration touse a retroreflector. These are described with reference to FIG. 15.FIG. 15 is a schematic view showing other examples of configurations forlight extraction

FIG. 15(1) shows an example employing a retroreflector 716. Theretroreflector 716 is used instead of the return mirror 712 in FIG. 12.In employing the retroreflector 716, an outgoing path and return pathare designed so as to deviate from each other in the length direction ofeach prism 711. In this case as well, an extraction mirror 715 isdisposed at the exit of the return path to extract a pulse-compressedsupercontinuum.

FIG. 15(2) is another example of the pulse compressor 7 that is madesmall-sized by employing two retroreflectors 718, 719. A supercontinuumfrom the waveguide 2 is refracted through the prism 111, returns byreflections on the first retroreflector 718, and then passes through theprism 711 again. It further returns to the prism 711 by reflections onthe second retroreflector 719, and passes through the prism 711 onceagain after reflections on the first retroreflector 718. It finallyreflects on the extraction mirror 715 to be extracted. Thisconfiguration, which is equal to use of two prisms (one prism pair) foroutgoing and return of light, is small sized as a whole.

Alternatively, it may be possible to provide a slight inclination anglebetween the outgoing path and return path in the prism pair compressor 7of FIG. 12, though it is not shown in a figure. In this configuration,the inclination angle is adequately chosen according to the beamdiameter and the optical path length of the supercontinuum so that thepulse-compressed supercontinuum can return to a different position, atwhich an extraction mirror is disposed to extract it.

If a light beam from the waveguide 2 is not linearly polarized, theconfiguration using the polarization beam splitter 713 shown in FIG. 12needs a polarizer to linearly polarize. This could cause a loss. Bycontrast, the configuration of FIG. 15(1)(2) and the configurationproviding the inclination angle are preferable because of no such aloss.

Other examples of the pulse compressor 7 are described next. Though theprism pair compressor 71 was used as the pulse compressor 7 in thesecond embodiment, other kinds of compression means are also usable.FIG. 16 shows another example of the pulse compressor 7.

As shown in FIG. 16(1), a pair of gratings (diffraction gratings) 72 maybe used for the pulse compressor 7. In the supercontinuum pulse shown inFIG. 16(1), the shorter wavelength wave L₁ having generated at a latertime point has the longer path length to the emission end of the gratingpair 72, compared with the longer wavelength wave L₂ having generated atan earlier time point. Therefore, disposition of each grating 72 withadequately-chosen tilt angles relating to the center wavelength of thesupercontinuum can make each wave temporally aligned (coherent),establishing the pulse compression.

Grating pairs are preferable for compression of a wide-bandsupercontinuum as in the embodiment because dispersion is possible forwider wavelength bands. Still it has a problem in efficiency. Whereasthe blaze angle is chosen according to the center wavelength of asupercontinuum in using a grating pair, diffraction loss may occur withdeviation from the blaze angle. Namely, such a loss as decrease of thepower with deviation from the center wavelength in the spectral powerdistribution may occur. As a result, it could happen that the wavelengthrange including the peak power capable of multiphoton excitation couldbe narrower, compared with the configuration using a prism pair.

As shown in FIG. 16(2), alternatively, a grism 73 also may be used asthe pulse compressor 7. The grism 73 has the structure where a combgrating 732 is disposed between a couple of prisms 731,731 arrangedplane-symmetrically. The couple of prisms 731 provides the optical pathlength difference according to wavelength as well as the prism pair, andhas the function of compensating the dispersion (spatial dispersion) bythe comb grating 732.

The configuration using the grism has the advantage of being smaller insize, compared with the grating pair. The problem of the diffractionloss, however, remains because a grating is still used.

As shown in FIG. 16(3), alternatively, a chirped mirror 74 may be usedas the pulse compressor 7. The chirped mirror 74, which includes amultilayer film formed on a substrate to reflect light, extends theoptical path length, because a light wave at a longer wavelengthreflects at a deer layer within the multilayer film.

Generally one chirped mirror only has group-delay dispersion as small asabout −50 fs², though pulse compression can be performed comparativelyeasier. Accordingly, the reflection has to be repeated, for instance,200 times or more to use a chirped mirror as the pulse compressor 7 inthis embodiment. Though the chirped mirror can have high reflectionrate, it has the disadvantage that the loss increases as a whole, if thereflection is repeated such many times.

As shown in FIG. 16(4), alternatively, an SLM (spatial light modulator)75 may be used as the pulse compressor 7. The SLM 75, which is theelement having an array of a lot of pixels where refraction indexes areindependently controlled as well as liquid crystal displays, is capableof secondary or other high-order dispersion compensation according to anapplied voltage. Transparent type and reflection type are known as SLM.A reflection type one, for instance, is adopted for pulse compression inthe configuration where a supercontinuum is spatially dispersed on thegrating 751, converted into the collimated light beam by a lens 752, andthen enters into each pixel, as shown in FIG. 16(4). The SLM 75 has theadvantages that waves in a desired wavelength band can be selectivelydelayed (group delayed), and that high-order dispersion compensationsare also possible. In general, group delay in one-time reflection isabout from several tens to two hundreds fs². For pulse compression inthis embodiment, therefore, it is preferable to use it additionally forparticularly selected wavelengths requiring group delay dispersion orhigh-order dispersion compensation. That is, it is preferable to use itfor a final adjustment.

A fiber having a specific structure or function may be used as the pulsecompressor 7 in the supercontinuum source in the embodiment. This pointis described next with reference to FIG. 17. FIG. 17 is a schematic viewshowing a further example of the pulse compressor 7.

A quite simple example using a fiber as the pulse compressor 7 is oneusing an anomalous dispersion fiber. Because a longer-wavelength wave isdelayed more in an anomalous dispersion fiber, pulse compression isenabled by making a supercontinuum propagate through an anomalous fiberhaving an adequate GVD and length.

As shown in FIG. 17(1), alternatively, a chirped fiber Bragg grating(CFBG) can be used as the pulse compressor 7. CFBGs are fibers whereportions having periodically-different refractive indexes are providedin cores along the length directions, functioning as diffractiongratings. Among these, it can be said that the CFBG 76 is the elementwhere the reflection position varies according to wavelength in order toachieve the chirped mirror function in the fiber. The CFBG 76 has thecharacteristic where a shorter-wavelength wave of incident lightreflects at a front position to return in the propagation direction, anda longer-wavelength wave reflects at a deeper position to return.

As shown in FIG. 17(2), alternatively, a dispersion decreasing fiber 77may be used as the pulse compressor 7 to carry out adiabatic solitoncompression. The dispersion decreasing fiber 77 has the characteristicwhere dispersion decreases gradually as light propagates. The dispersiondecreasing fiber 77 has the structure where, for instance, both of thecore diameter and the clad diameter decrease to the light propagationdirection. Fundamental soliton is the stable state where anomalousdispersion and self-phase modulation are in balance in a fiber. Use ofthe dispersion decreasing fiber 77 enables pulse compression as afundamental soliton is maintained, because the fundamental solitoncondition can be changed continuously according to the distance. Theadiabatic soliton compression has the advantages of high compressionrate and superior sech-type pulse quality.

As shown in FIG. 17(3), alternatively, a comb-like dispersion profiledfiber 78 may be used as the pulse compressor 7. Comb-like dispersionprofiled fibers are fibers where dispersion shift fibers (DSF) andsingle mode fibers (SMF) are arranged alternately. The comb-likedispersion profiled fiber 78 has the advantage that required compressioncharacteristics are easily realized by commercially available fibers,though it has the problem of the loss due to many connections therein.

As described, the supercontinuum source in the embodiment uses thenonlinear fiber the as the waveguide 2. Use of a fiber as the pulsecompressor 7 has the merit that affinity among the elements is enhanced,and thus handlings and adjustments are made easier, because generationand compression of a supercontinuum can be completed within the fibersystem.

Still, when a fiber is used for pulse compression, attention must bepaid so that no undesired high-order nonlinear effects can occur byincidence of a high energy supercontinuum. Nevertheless once a flatsupercontinuum has been generated successfully over a wide band, afurther nonlinear optical effect could occur in pulse compression,causing vacant bands, extreme drops, etc. in the spectrum. This shouldbe avoided. In this respect, such group delay dispersion elements asprism pair, grating, grism and chirped mirror are preferable becausethose are kinds of spatial elements, being capable of enlarging beamdiameters freely, whereas beam diameters for fibers are only micrometerorder. Such spatial elements are also preferable in designing becausenonlinear optical effects hardly occur.

Use of a fiber needs caution for its thermal damage by self-convergenceor the like, because peak power increases by pulse compression. Toprevent fiber damage it is effective to adopt a large mode area (LMA)fiber or hollow-core photonic bandgap fiber (PBF). Those are alsoeffective to prevent spectral distortion that could be caused by anundesired nonlinear optical effect.

LMA photonic crystal fibers (LMA-PCF) capable of single mode propagationin wide bands are already commercially available. Though currently thereis no LMA-PCF with anomalous dispersion in the whole wavelength rangefor the present invention, it may be used if developed. Though hollowcore PBFs have the feature of essentially no optical nonlinearity due toits hollow core structure, those which are commercially available areall low dispersion fibers. In case one providing sufficient dispersionis developed, however, it may be used as well.

EXPLANATION OF SIGNS

-   1 Pulse Oscillator-   2 Waveguide-   3 Polarization Control Element-   4 Supercontinuum Source-   5 Optical System-   51 Lens-   52 Dichroic Mirror-   53 Scanning Mirror Unit-   531 Scanning Mirror-   54 Objective Lens-   6 Detector-   61 Imaging Device-   62 Filter-   7 Pulse Compressor-   71 Prism Pair Compressor-   711 Prism-   712 Return Mirror-   713 Polarization Beam Splitter-   714 Quarter Wavelength Plate-   72 Grating-   73 Grism-   74 Chirped Mirror-   75 Spatial Light Modulator-   76 CFBG-   77 Dispersion Decreasing Fiber-   78 Comb-like Dispersion Profiled Fiber

What is claimed:
 1. A supercontinuum source to generate and emit asupercontinuum, comprising: a pulse oscillator oscillating an ultrashortlight pulse, and a waveguide converting the oscillated ultrashort lightpulse from the pulse oscillator into a supercontinuum by a nonlinearoptical effect, and then emitting, wherein the waveguide has aconversion characteristic where the ultrashort light pulse is convertedinto the supercontinuum having a spectrum continuous in a wavelengthband width of at least 200 nm included in the wavelength range from 850to 1550 nm excluding the wavelength range from 1430 to 1550 nm andhaving a wavelength flatness within 3 dB in the wavelength band width ofat least 200 nm included in the wavelength range from 850 to 1550 nmexcluding the wavelength range from 1430 to 1550 nm, and the pulseoscillator and the waveguide are operable to emit the supercontinuumwith a peak power within 1 to 100 kW enabling multiphoton excitation ofan object on an irradiated plane.
 2. The supercontinuum source asclaimed in claim 1, wherein the ultrashort light pulse has a pulse widthnot more than 1 ps and a center wavelength in the wavelength range from1000 to 1100 nm.
 3. The supercontinuum source as claimed in claim 1,wherein the waveguide has the conversion characteristic where thetemporal shift of wavelength of the supercontinuum is continuous in onepulse.
 4. The supercontinuum source as claimed in claim 1, wherein thewaveguide is a fiber with normal dispersion in the wavelength range from850 to 1550 nm excluding the wavelength range from 1430 to 1550 nm. 5.The supercontinuum source as claimed in claim 4, wherein the centerwavelength of the ultrashort light pulse is in the range of plus andminus 50 nm to the peak wavelength of the group velocity dispersionspectrum of the fiber with normal dispersion.
 6. The supercontinuumsource as claimed in claim 1, further comprising a pulse compressor toincrease the peak power of the supercontinuum by compressing thesupercontinuum emitted from the waveguide.
 7. The supercontinuum sourceas claimed in claim 3, further comprising a pulse compressor to increasethe peak power of the supercontinuum by compressing the supercontinuumemitted from the waveguide.
 8. A supercontinuum source to generate andemit a supercontinuum, comprising a pulse oscillator oscillating anultrashort light pulse, and a waveguide converting the oscillatedultrashort light pulse from the pulse oscillator into a supercontinuumby a nonlinear optical effect and then emitting, wherein the waveguidehas a conversion characteristic where the supercontinuum has a spectrumcontinuous in a band width of at least 200 nm included in the wavelengthrange from 850 to 1550 nm excluding the wavelength range from 1430 to1550 nm and has a wavelength flatness within 3 dB in the wavelength bandwidth of at least 200 nm included in the wavelength range from 850 to1550 nm excluding the wavelength range from 1430 to 1550 nm, and a pulsecompressor is provided to increase the peak power of the supercontinuumemitted from the waveguide by pulse compression thereof.
 9. A method forgenerating and emitting a supercontinuum, comprising oscillating anultrashort light pulse by a pulse oscillator, making the ultrashortlight pulse from the pulse oscillator enter into a waveguide, andconverting the ultrashort light pulse into a supercontinuum by anonlinear optical effect in the waveguide, and then emitting therefrom,wherein the waveguide converts the ultrashort light pulse into thesupercontinuum having a spectrum continuous in a band width of at least200 nm included in the wavelength range from 850 to 1550 nm excludingthe wavelength range from 1430 to 1550 nm and having a wavelengthflatness within 3 dB in the wavelength band width of at least 200 nmincluded in the wavelength range from 850 to 1550 nm excluding thewavelength range from 1430 to 1550 nm, further comprising emitting thesupercontinuum with a peak power within 1 to 100 kW to enablemultiphoton excitation of an object on an irradiated plane.
 10. Themethod for generating and emitting a supercontinuum as claimed in claim9, wherein the ultrashort light pulse oscillated from the pulseoscillator has a pulse width not more than 1 ps and the centerwavelength in the wavelength range from 1000 nm to 1100 nm.
 11. Themethod for generating and emitting a supercontinuum as claimed in claim9, wherein the waveguide converts the ultrashort light pulse into thesupercontinuum where the temporal shift of wavelength is continuous. 12.The method for generating and emitting a supercontinuum as claimed inclaim 9, wherein the waveguide is a fiber with normal dispersion in thewavelength range from 850 to 1550 nm excluding the wavelength range from1430 to 1550 nm.
 13. The method for generating and emitting asupercontinuum as claimed in claim 12, wherein the center wavelength ofthe ultrashort light pulse is in the range of plus and minus 50 nm tothe peak wavelength of the group velocity dispersion spectrum of thefiber with normal dispersion.
 14. The method for generating and emittinga supercontinuum as claimed in claim 9, further comprising increasingthe peak power of the supercontinuum emitted from the waveguide by pulsecompression thereof.
 15. The method for generating and emitting asupercontinuum as claimed in claim 11, further comprising increasing thepeak power of the supercontinuum emitted from the waveguide by pulsecompression thereof.
 16. A multiphoton excitation fluorescencemicroscope comprising the supercontinuum source as claimed in claim 1,further comprising an optical system to irradiate an object with thesupercontinuum emitted from the waveguide, and a detector to detect afluorescence emitted when the object is subjected to multiphotonexcitation by the supercontinuum.
 17. The supercontinuum source asclaimed in claim 6, wherein the pulse compressor is a prism paircompressor comprising a pair of prisms where a light wave at a longerwavelength has a longer overall propagation length.
 18. Thesupercontinuum source as claimed in claim 8, wherein the pulsecompressor is a prism pair compressor comprising a pair of prisms wherea light wave at a longer wavelength has a longer overall propagationlength.
 19. A multiphoton excitation method for causing multiphotonexcitation on a fluorescent substance by a supercontinuum, comprising anoscillation process where an ultrashort light pulse is oscillated by apulse oscillator, a conversion process where the ultrashort light pulseoscillated from the pulse oscillator is made enter into the waveguide,converted into the supercontinuum by a nonlinear optical effect in thewaveguide, and then emitted, a compression process where the peak powerof the emitted supercontinuum is increased by pulse compression, and anirradiation process where all or a part of the supercontinuum which peakpower has been increased in the compression process is applied to thefluorescent substance to cause multiphoton excitation, wherein in theconversion process the ultrashort light pulse is converted in thewaveguide into the supercontinuum having a spectrum continuous in a bandwidth of at least 200 nm included in the wavelength range from 850 to1550 nm excluding the wavelength range from 1430 to 1550 nm.
 20. Thesupercontinuum source as claimed in claim 6, wherein the pulsecompressor is a prism pair compressor comprising plural pairs of prismswhere a light wave at a longer wavelength has a longer overallpropagation length, further comprising a mirror at a position where thesupercontinuum from the pairs of the prisms is returned thereto, makingthe supercontinuum pass through the pairs of the prisms plural times.21. The supercontinuum source as claimed in claim 7, wherein the pulsecompressor is a prism pair compressor comprising plural pairs of prismswhere a light wave at a longer wavelength has a longer overallpropagation length, further comprising a mirror at a position where thesupercontinuum from the pairs of the prisms is returned thereto, makingthe supercontinuum pass through the pairs of the prisms plural times.22. The supercontinuum source as claimed in claim 8, wherein the pulsecompressor is a prism pair compressor comprising plural pairs of prismswhere a light wave at a longer wavelength has a longer overallpropagation length, further comprising a mirror at a position where thesupercontinuum from the pairs of the prisms is returned thereto, makingthe supercontinuum pass through the pairs of the prisms plural times.